The field of quantum acoustics studies high frequency sounds generated at low temperatures such that quantum mechanical effects become relevant. The studies mainly revolves around propagating quantized sound waves, or phonons, a collective excitation of atoms in solids or liquids. In quantum acoustics, the engineering and design tools described by circuit quantum acoustodynamics (cQAD) are used to develop quantum acoustic devices that are coupled to superconducting qubits. cQAD enabled the demonstrations of quantum ground state cooling mechanical objects, generating mechanical Fock-states, and Schrödinger cat states of motion. This makes quantum acoustic devices appealing candidates for applications such as quantum metrology, information processing, and quantum memory. This thesis focuses on the coupling between a planar superconducting transmon qubit and a high-overtone bulk acoustic resonator (HBAR) and explore its possibilities. Here,experimental demonstrations are shown where the transmon is used to drive the HBAR into a phonon lasing state making it a superconducting single-atom phonon laser. Furthermore, the transmon-HBAR device is used to probe the nature of ghost modes observed in strongly driven nonlinear systems.@en

Cavity optomechanics studies the interaction between mechanical resonators and optical cavities through radiation pressure forces and aims to harness this interaction for applications in the areas of high precision metrology, tests of fundamental quantum mechanics, or quantum information processing. For the most ambitious of these applications it is necessary that the mechanical resonator has a sufficiently high mechanical quality factor such that it can undergo at least a few coherent oscillations before interacting with incoherent thermal phonons. Furthermore, the optomechanical coupling must be large enough to make the interaction between optics and mechanics probable and, ideally, deterministic. This work pursues both goals using a thin membrane in the middle (MIM) of an optical cavity. This is a common configuration in cavity optomechanics but most experiments to date have lowmechanical quality factors and optomechanical couplings.@en

In cavity optomechanics, optical fields are coupled to the displacement of mechanical resonators. While it is interesting to study fundamental aspects of this interaction, it is the ability to link this mechanical displacement to various other degrees of freedom that inspires many applications in the field. In these applications, the mechanical resonator can be used as a handle to an external influence for the purpose of sensing, but also as a transducer between two otherwise detached degrees of freedom. The latter approach is the focus of this work. A particularly interesting regime for such a transduction process is between a few-gigahertz microwave tone and optical photons at telecom wavelengths around 1550 nm, connecting the operating regimes of long-range telecommunication with that of superconducting quantum nodes. Bridging the gap between these domains is an essential step towards any size of quantum network based on superconducting nodes, as the losses encountered in microwave transmission lines prohibits the connection of such nodes over length scales extending beyond a few meters. As such, a transducer between the few-gigahertz and optical telecom domains would enable the use of low-loss optical channels to connect remote superconducting nodes, given that the quantum information is preserved throughout the conversion process. One approach of realizing such a converter makes use of a gigahertz-frequency mechanical mode as the transducing element, which is coupled to the optical telecom domain using the optomechanical interaction and to the microwave domain using the electromechanical interaction. In this work, we aim to unify both of these interactions in a single device by designing and fabricating optomechanical devices from the III/V semiconductors, which, alongside their good optical properties, are also piezoelectric. In Chapter 2, we motivate our material choice and introduce the relevant properties of the materials, as well as their impact on the fabrication process. Following the material discussion, we then set out in Chapter 3 to realize a microwave-to-optics converter made of an optomechanical crystal in gallium arsenide, which we resonantly couple to a interdigital transducer using surface-acoustic-waves. With this device, we demonstrate the first microwave-to-optics conversion using a mechanical mode with an average number of thermal excitations below one. As well as verifying the coherence of the conversion process, our experiments also highlight the limitations arising from the material choice due to absorption-induced incoherent heating of the mechanical mode. The material choice itself then becomes the central topic of Chapter 4, where we opt for a gallium phosphide, a relative of gallium arsenide, which prominently features a larger bandgap. With this material we show non-classical correlations between photons an phonons. We enter a a regime that was previously inaccessible to devices made from piezoelectric materials, a promising step towards realizing the noise-requirements for microwave-to-optics converters. In Chapter 5, we use the insights from the two previous chapters to design a new type of electro-opto-mechanical resonator, specifically aimed at microwave-to-optics conversion. We miniaturize the electromechanical interface and use strongly coupled mechanical resonators for the transfer of excitations between the microwave and mechanical mode. We fabricate and characterise initial devices as well as demonstrate the validity of the functioning principle. Finally in Chapter 6, we reflect on the results of the previous chapters and highlight some potential advantages of other approaches. @en

In this thesis, we explore the physics of optical singularities. We investigate them in light waves propagating randomly in a planar nanophotonic chip. With a custom-built nearfield microscope, we map the electromagnetic field resulting from the interference of these light waves. Our technique gives access to the full vectorial and complex nature of such an electromagnetic field, with subwavelength resolution. The resulting information allows us to precisely pinpoint and characterize the multitude of singularities that arise in the random light field. We detect phase singularities in the Cartesian components of light’s vector field, i.e., points where the phase of the field components is undetermined and it circulates in a vortical flow around them (Part II).Moreover, we identify polarization singularities, e.g., C points: locations where the vector of light’s electric field describes a perfect circle in time (Part III)... @en

The work in this thesis focuses on potential ways to bring a macroscopic mechanical resonator in a classical environment into the quantum regime with integrated cavity optomechanics...@en

Cavity Optomechanics is a field employing optical control over mechanical oscillators with a variety of possible applications for sensing, optical signal processing as well as quantum technologies. Such systems are attractive because they are often fully engineerable and can be tailored to specific applications by allowing a large range of frequencies and the usage of a number of host materials. The following work explores one such application of a high frequency, ultra-long lived mechanical mode as a quantum memory which can be read out on-demand via an optical interface.@en

Quantum communication refers to the field of science that studies the ability to connect separated quantum devices via coherent channels, i.e. via buses that maintain coherently the information encoded. The importance of the task is on multiple levels: from secure communications to scaling quantum computers. The first one can be of fundamental importance in moments like government elections, or banks transaction, but even to secure the right of privacy of individuals. The latter could open the way to, for example, faster and more precise solutions to problems in chemistry (for drug development), or material science (more environmentally friendly solar cells). At this moment, quantum communication and computing are in a similar stage as of the early computers: the machines are with very little connectivity and require large spaces and specialists to be operated. In a few years, we can expect these systems to be interconnected more and more, scaled, and made easier to use. In this thesis, we present work done to create quantum channels using high frequency mechanical oscillators. In chapter 1 we present recent progress in the field of quantum communication done with several types of systems, both in the long and short distance. We also introduce how high frequency mechanical oscillators could play an important role in this research area. We discuss the current challenges and limitations and possible future developments. In chapter 2 we perform a optomechanical quantum teleportation. In this work, we teleport a polarization encoded telecom photon onto a quantum memory, made by two single mode mechanical oscillators in a dual-rail configuration. This work is a step towards entanglement swapping (also referred to as ’teleportation of entangled state’) and represents a proof of principle towards quantum repeaters (using the scheme proposed by Duan, Lukin, Cirac, and Zoller - DLCZ scheme). In chapter 3 we report the first experiment done with the multimode mechanical devices. These devices are formed by a single mode optomechanical cavity coupled to a single-mode mechanical waveguide (ended with a phononic mirror). We show that the non-classical information created in the optomechanical cavity can be guided on chip in the mechanical waveguide, using as witness the cross-correlation between the scattered photons. However, the non-uniform spacing between the mechanical modes severely lowers the maximum value of the non-classical correlation measured. This was greatly improved with the new design of the device. With this design, we are able to entangle two traveling phonons in the mechanical waveguide, shown in chapter 4. In this way, we show that the traveling phonons can be used to distribute quantum entanglement on-chip, a first step towards connecting quantum devices on a short scale. In chapter 5, we measure in time the frequency jitter of two spectrally close mechanical modes of the same device. We demonstrate that the frequency diffusion of the modes is not correlated in time, and so the coherence length of the traveling information will ultimately be limited by the jitter. This result shows the importance of performing a detailed study on the surface defects. Lastly, in chapter 6 we summarize the findings of these experiments and we discuss the future developments of the field.@en

The distribution of multiple entangled pairs plays a vital role in various applications, such as quantum metrology, blind quantum computing, quantum key distribution, and quantum teleportation. While qubit-based protocols exist, they are often limited by short coherence times in quantum memories and the need for individual pair generation, leading to inefficiencies and reduced fidelity. To address these challenges, this thesis focuses on the development and analysis of a comprehensive theoretical framework for satellite-based entanglement distribution using high-dimensional qudits. By employing high-dimensional qudit encoding, which enables the simultaneous entanglement of multiple pairs, we aim to enhance the capacity and efficiency of entanglement distribution schemes. The objectives of this research are twofold, first, to compare the performance of the qudit-based protocol with a similar qubit-based protocol, particularly in a satellite-based context, and second, to investigate the feasibility and potential advantages of employing high-dimensional qudit encoding for near-term long-range entanglement distribution. The theoretical framework encompasses various elements, including a single photon pair source, lossy transmission, mapping of qudit states to qubit memories, and heralding measurements. Through various simulations, we evaluate the impact of different parameters on the performance of the proposed protocol. Our findings reveal that high-dimensional encoding significantly alleviates memory requirements, making it suitable for current technologies with low memory coherence times. Multiplexing techniques further enhance the achievable distances and transmission rates, highlighting their importance for realistic implementations. However, we note that higher-dimensional protocols introduce additional experimental challenges and errors, warranting further research and optimisation. Furthermore, we explore the effect of reducing dark count rates in detectors and analyse its influence on the overall performance. Our investigations underscore the need for substantial detector improvements, as current dark count probabilities are on the same order of magnitude as channel losses. By advancing our understanding of satellite-based entanglement distribution using qudits, this research contributes to the development of practical and secure quantum communication networks. The results presented herein lay the groundwork for future advancements in quantum communication technologies, fostering the realisation of global-scale quantum networks.